let s1 = 'hello world';
let s2 = s1.substring(2);


/*
    let s1 = new String('hello world');
    let s2 = s1.substring(2);
    s1 = null;
*/

let a = '25';
let b1 = Number(a);
console.log(typeof b1);   // number

let b2 = new Number(a);
console.log(typeof b2);   // object
